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CGESV Example Program in Fortran
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* =============================================================================
*
* CGESV Example.
* ==============
*
* The program computes the solution to the system of linear
* equations with a square matrix A and multiple
* right-hand sides B, where A is the coefficient matrix:
*
* ( 1.23, -5.50) ( 7.91, -5.38) ( -9.80, -4.86) ( -7.32, 7.57)
* ( -2.14, -1.12) ( -9.92, -0.79) ( -9.18, -1.12) ( 1.37, 0.43)
* ( -4.30, -7.10) ( -6.47, 2.52) ( -6.51, -2.67) ( -5.86, 7.38)
* ( 1.27, 7.29) ( 8.90, 6.92) ( -8.82, 1.25) ( 5.41, 5.37)
*
* and B is the right-hand side matrix:
*
* ( 8.33, -7.32) ( -6.11, -3.81)
* ( -6.18, -4.80) ( 0.14, -7.71)
* ( -5.71, -2.80) ( 1.41, 3.40)
* ( -1.60, 3.08) ( 8.54, -4.05)
*
* Description.
* ============
*
* The routine solves for X the system of linear equations A*X = B,
* where A is an n-by-n matrix, the columns of matrix B are individual
* right-hand sides, and the columns of X are the corresponding
* solutions.
*
* The LU decomposition with partial pivoting and row interchanges is
* used to factor A as A = P*L*U, where P is a permutation matrix, L
* is unit lower triangular, and U is upper triangular. The factored
* form of A is then used to solve the system of equations A*X = B.
*
* Example Program Results.
* ========================
*
* CGESV Example Program Results
*
* Solution
* ( -1.09, -0.18) ( 1.28, 1.21)
* ( 0.97, 0.52) ( -0.22, -0.97)
* ( -0.20, 0.19) ( 0.53, 1.36)
* ( -0.59, 0.92) ( 2.22, -1.00)
*
* Details of LU factorization
* ( -4.30, -7.10) ( -6.47, 2.52) ( -6.51, -2.67) ( -5.86, 7.38)
* ( 0.49, 0.47) ( 12.26, -3.57) ( -7.87, -0.49) ( -0.98, 6.71)
* ( 0.25, -0.15) ( -0.60, -0.37) (-11.70, -4.64) ( -1.35, 1.38)
* ( -0.83, -0.32) ( 0.05, 0.58) ( 0.93, -0.50) ( 2.66, 7.86)
*
* Pivot indices
* 3 3 3 4
* =============================================================================
*
* .. Parameters ..
INTEGER N, NRHS
PARAMETER ( N = 4, NRHS = 2 )
INTEGER LDA, LDB
PARAMETER ( LDA = N, LDB = N )
*
* .. Local Scalars ..
INTEGER INFO
*
* .. Local Arrays ..
INTEGER IPIV( N )
COMPLEX A( LDA, N ), B( LDB, NRHS )
DATA A/
$ ( 1.23,-5.50),(-2.14,-1.12),(-4.30,-7.10),( 1.27, 7.29),
$ ( 7.91,-5.38),(-9.92,-0.79),(-6.47, 2.52),( 8.90, 6.92),
$ (-9.80,-4.86),(-9.18,-1.12),(-6.51,-2.67),(-8.82, 1.25),
$ (-7.32, 7.57),( 1.37, 0.43),(-5.86, 7.38),( 5.41, 5.37)
$ /
DATA B/
$ ( 8.33,-7.32),(-6.18,-4.80),(-5.71,-2.80),(-1.60, 3.08),
$ (-6.11,-3.81),( 0.14,-7.71),( 1.41, 3.40),( 8.54,-4.05)
$ /
*
* .. External Subroutines ..
EXTERNAL CGESV
EXTERNAL PRINT_MATRIX, PRINT_INT_VECTOR
*
* .. Executable Statements ..
WRITE(*,*)'CGESV Example Program Results'
*
* Solve the equations A*X = B.
*
CALL CGESV( N, NRHS, A, LDA, IPIV, B, LDB, INFO )
*
* Check for the exact singularity.
*
IF( INFO.GT.0 ) THEN
WRITE(*,*)'The diagonal element of the triangular factor of A,'
WRITE(*,*)'U(',INFO,',',INFO,') is zero, so that'
WRITE(*,*)'A is singular; the solution could not be computed.'
STOP
END IF
*
* Print solution.
*
CALL PRINT_MATRIX( 'Solution', N, NRHS, B, LDB )
*
* Print details of LU factorization.
*
CALL PRINT_MATRIX( 'Details of LU factorization', N, N, A, LDA )
*
* Print pivot indices.
*
CALL PRINT_INT_VECTOR( 'Pivot indices', N, IPIV )
STOP
END
*
* End of CGESV Example.
*
* =============================================================================
*
* Auxiliary routine: printing a matrix.
*
SUBROUTINE PRINT_MATRIX( DESC, M, N, A, LDA )
CHARACTER*(*) DESC
INTEGER M, N, LDA
COMPLEX A( LDA, * )
*
INTEGER I, J
*
WRITE(*,*)
WRITE(*,*) DESC
DO I = 1, M
WRITE(*,9998) ( A( I, J ), J = 1, N )
END DO
*
9998 FORMAT( 11(:,1X,'(',F6.2,',',F6.2,')') )
RETURN
END
*
* Auxiliary routine: printing a vector of integers.
*
SUBROUTINE PRINT_INT_VECTOR( DESC, N, A )
CHARACTER*(*) DESC
INTEGER N
INTEGER A( N )
*
INTEGER I
*
WRITE(*,*)
WRITE(*,*) DESC
WRITE(*,9999) ( A( I ), I = 1, N )
*
9999 FORMAT( 11(:,1X,I6) )
RETURN
END
Parent topic: CGESV Example